Jerilyn has a $10 coupon and a 15% discount coupon for her favorite store. The store has a policy that only one coupon may be used per purchase. When is it best for Jerilyn to use the $10 coupon, and when is it best for her to use the 15% discount coupon?

It is best for Jerilyn to use the $10 coupon when the value of the purchase is equal or lower than $66.67, and it is best to use the $10 coupon when the value of the purchase is greater than $66.67

Explanation:

Assume the value of the purchase is P.

Then 15% of P is 0.15P.

To obtain the maximum benefit from the 15% coupon, Jerilyn should use it when the discount from it is greater than the discount from the $10 coupon. This is:

0.15P > $10

Divide both sides by 0.15:

P > $10 / 0.15

P > $66.67

If the value of the purchase is equal to $66.67 the total discount with any cuopon are equal; if it is lower than $66.67, the discount of the $10 coupon is greater.

Thus, you conclude that for a $66.67 purchase she should use the $10 cuopon and for a purchase greater than $66.67 she should used the 15% cuopon.